The purpose of such a detector is to suitably sequence the injection of gasoline into the cylinders of the engine by initializing the injection into a predetermined cylinder, called the number one or first cylinder.
A first cylinder detector is indispensable both in engines with dynamic computerized ignition and in engines with static ignition, also by computer, or, in other words, in all engines with ignition by computer, which for its injection function must be initialized.
The present invention particularly relates to a first cylinder detector including a unipolar Hall-effect pickup, associated with a permanent magnet and arranged to detect the passage of a target of soft magnetic material, i.e., the material without a permanent magnetization zone, which upon passage past the pickup and the permanent magnet, loops the field lines of the magnet and the pickup. A target, such as a boss, is formed on a pulley which is fixed to the camshaft of the engine so as to correspond to top dead center (TDC) of the first cylinder piston.
Upon the passage of the target past the detector, the sensitive zone of the Hall effect pickup is thus subjected to a variation in magnetic flux that is to be detected.
In fact, this magnetic flux variation cannot be detected if it is inadequate in proportion to a permanent leakage flux between the pickup and the permanent magnet.
In other words, unless the magnet is to be shifted exaggeratedly relative to the pickup, the magnetic induction in the absence of the target may be too high for a change in the state of the Hall circuit upon passage of the target to be detectable.
Again, in other words, this kind of first cylinder detector is reliable only for very small air gaps between the target and the pickup of less than approximately 0.2 mm.
To increase this air gap size, it has already been proposed that the pickup be doubled, so that two such pickups are associated with one permanent magnet. The output signal of the pickup closest to the path of the target is modulated, while the output signal of the other is not, and by addition of the output signals, a target passage with an air gap that may reach approximately 1 mm is detectable. However, this is a tedious provision because it requires two pickups of the same sensitivity, which is especially difficult to achieve.
The present invention seeks to propose a solution to this problem.